Field
The disclosure relates generally to ion exchanged thin glass laminates, and more particularly ion exchanged thin glass laminates having a low coefficient of thermal expansion (CTE) ion-exchanged glass outer clad glass layer(s) laminated onto a high CTE glass inner core glass layer for creating compressive stresses in the outer clad glass layer(s), and more particularly to such laminates that have been strengthened via ion exchange, and more particularly to glass fusion forming and laminating processes and glass compositions for making such laminates, which may be used in automotive, aviation, architectural, appliance, display, touch panel, and other applications where a thin, strong, scratch resistant glass product is advantageous.
Technical Background
Glass articles, such as cover glasses, glass backplanes and the like, are employed in both consumer and commercial electronic devices such as LCD and LED displays, computer monitors, automated teller machines (ATMs) and the like. Some of these glass articles may include “touch” functionality which necessitates that the glass article be contacted by various objects including a user's fingers and/or stylus devices and, as such, the glass must be sufficiently robust to endure regular contact without damage. Moreover, such glass articles may also be incorporated in portable electronic devices, such as mobile telephones, personal media players, and tablet computers. The glass articles incorporated in these devices may be susceptible to damage during transport and/or use of the associated device. Accordingly, glass articles used in electronic devices may require enhanced strength to be able to withstand not only routine “touch” contact from actual use, but also incidental contact and impacts which may occur when the device is being transported.
Various processes may be used to strengthen glass articles, including chemical tempering, thermal tempering, and lamination. Lamination mechanical glass strengthening is the primary mechanism that is responsible for the strength of Corelle® dinnerware, for example, that enables the dinnerware to withstand repeated damage from cutlery and general handling. Such dinnerware is made by thermally bonding or laminating three layers of glass, namely a glass core or center layer having a relatively high coefficient of thermal expansion (CTE) surrounded by two outer clad or skin layers having a relatively low CTE. Upon cooling of the laminate following thermal bonding of the clad glass layers to the outer surfaces of the core glass layer, the relatively high CTE of the core glass layer (compared to the CTE of the clad glass layers) causes the core glass layer to contract or shrink more than the clad glass layers. This causes the core glass layer to be in a state of tension and the clad glass layers to be in state of compression. The compressive stresses in the clad glass layers inhibit fracture formation and fracture propagation in the clad glass layers, thereby strengthening the glass laminate compared to clad glass that is not under compressive stresses. The laminate may also be thermally tempered to increase the compressive stress in the clad glass. The stress profile in such a mechanically strengthened glass laminate is schematically illustrated by the solid line A in FIG. 1. FIG. 1 plots the level of stress in the glass laminate (compressive stress − and tensile stress +) at different depths along the thickness of the glass laminate. The right and left sides in FIG. 1 correspond to the opposing outer surfaces of the glass laminate. The area designated by arrows 1 in FIG. 1 represents the core glass layer, and the areas designated by 2 arrows represent the clad glass layers. As can be seen in FIG. 1, the core glass is in a state of tensile stresses and the clad glass is in a state of compressive stresses.
Ion-exchange chemical strengthening is used by, for example, Corning Incorporated to strengthen Corning® Gorilla® glass. Gorilla glass is currently used as a cover glass for displays and touch screens in electronic devices such as smart phones, tablet computers and televisions. An example of an ion-exchange process is provided by U.S. patent Ser. No. 12/537,393, entitled STRENGTHENED GLASS ARTICLES AND METHODS OF MAKING, filed on Aug. 7, 2009, the disclosure of which is hereby incorporated by reference. In an ion exchange strengthening process, ions in the surface layer of the glass are replaced by, or exchanged with, larger ions in a bath solution (such as a salt bath) having the same valence or oxidation state. The glass being ion exchanged may be an aluminosilicate glass. Ions in the surface layer of the glass and the larger ions in the bath are monovalent alkali metal cations, such as Li+ (when present in the glass), Na+, K+, Rb+, and Cs+. Alternatively, monovalent cations in the surface layer may be replaced with monovalent cations other than alkali metal cations, such as Ag+ or the like. The stress profile of a glass sheet that has been chemically strengthened using an ion exchange process is illustrated by the dashed line B in the plot of FIG. 1. The area designated by arrows 3 in FIG. 1 represents a sheet of chemically strengthened glass. As can be seen in FIG. 1, the outer most or near surface portions of the glass sheet are in a state of compression and the central portion of the glass sheet is in a state of tension.
In both types of strengthened glass illustrated in FIG. 1, the compressive stress extends to a certain depth below the outer surface of glass, which depth is commonly referred to the depth of layer. The level of compressive stress in this outer most layer is commonly referred to as the compressive stress. Mechanical performance of strengthened glass is directly related to the shape of the stress profile, e.g. the depth of layer and the magnitude of the compressive stress present at a particular depth. The greater the depth of the compressive layer and the greater the compressive stress in the glass, then the stronger and more fracture resistant and fracture propagation resistant the glass will be. A high compressive stress in the near surface regions of ion exchanged chemically strengthened glass product inhibits fracture formation in the surface of the glass (providing scratch resistance) and inhibits fracture propagation from any fractures defects that exist or are created in the surface of the glass. Once a fracture propagates all the way through the near surface region of the glass that is under compressive stress (i.e. through the depth of layer) and the tip of the crack reaches the inner portion of the glass sheet or laminate that is under tension, then the crack quickly propagates through the glass resulting in a sudden failure and the glass product shatters.
Thin glasses, less than 2 mm thick, cannot be effectively tempered thermally. Such thin glasses must either be laminated to take advantage of expansion differential or they must be ion-exchanged to develop adequate levels of surface compression and depth of layer. Silicate glasses that can be laminated do not develop sufficient surface compressive stress due to limited amount of expansion differential between the clad glass and core glass. The most compressive stress that is theoretically possible by laminating silicate glasses is in a range from about 275 MPa to about 350 MPa with non-zero depth of layer. This level of compression is inadequate for some applications, i.e. for mitigating impact stresses in day to day use. Gorilla glass, for example, enjoys a surface compression approaching 800 MPa. Hence, laminated glass needs to be augmented with ion-exchange process to achieve surface compression approaching 800 MPa.